Bent and tempered glass is used extensively for vehicle side and rear windows to provide good resistance to breakage as well as providing an aesthetically appealing shape that complements the configuration of the vehicle openings and the overall design of the vehicle. In addition, those surfaces of a glass sheet within the viewing area must be free from mars and defects which would tend to interfere with clear vision through the window.
In order to perform the bending and tempering, the sheet glass must be heated to its deformation point of about 1050.degree. F. to 1200.degree. F. and then bent to the required shape before being rapidly cooled by an air spray in order to temper the glass. Tempering greatly increases the mechanical strength of the glass and its resistance to breakage as well as causing the glass to break into relatively small, dull pieces when broken instead of large slivers as in the case of untempered glass.
U.S. Pat. No. 4,282,026, issued Aug. 4, 1981, entitled "Apparatus for Bending and Tempering Glass", is related to and the entire disclosure is incorporated by reference in the present application. The patent discloses a glass transport apparatus including a furnace with a housing that defines a heating chamber in which glass is heated during conveyance along a roller hearth conveyor. An upwardly opening lower housing portion and a fixed roof of the furnace housing cooperates with vertically movable side doors to define the heating chamber. Lower ends of the doors cooperate with the upper ends of side walls on the lower housing portion to define side slots through which ends of elongated conveyor rolls project outwardly from the heating chamber. Continuous drive loops in the form of either chains or solid steel belts support the ends of the conveyor rolls and are slidably driven over external support surfaces extending alongside the slots to support and frictionally drive the rolls. A vacuum holder is positioned within the heating chamber above the conveyor rolls and has a downwardly facing inner surface with restricted openings spaced thereover. A vacuum is drawn within the restricted openings to receive the heated sheet of glass from the conveyor and support the glass above the conveyor in preparation for bending.
Prior patents disclose glass sensors which sense the position of individual glass sheets conveyed by glass transport mechanisms within a glass processing environment without requiring mechanical engagement of the glass sheets. For example, the U.S. patent of Rahrig et al. U.S. Pat. No. 3,372,016 discloses photoelectric cells in the cooling area of glass sheet tempering and conveying apparatus to sense the glass sheets. Likewise, the U.S. patent of Stickel et al., U.S. Pat. No. 3,459,529 discloses photoelectric cells within the bending area of a glass sheet bending apparatus to sense the glass sheets. As disclosed in the U.S. patent of Carson et al., U.S. Pat. No. 3,522,029, photocells are located adjacent the heating area of glass sheet reshaping apparatus to sense the glass sheets.
The previously mentioned U.S. patent entitled "Apparatus for Bending and Tempering Glass" discloses photoelectric sensors which sense moving glass sheets and which are located in a heating chamber which heats the glass sheets as they are conveyed by a conveyor therethrough.
Glass sheets moving within a glass processing environment such as a glass furnace must be accurately sensed to precisely locate the position of the glass sheets so that subsequent, automatically controlled processing and/or positioning of the glass sheets can be performed without damage to either the glass sheets or the glass processing apparatus. For example, the positions of a pair of glass sheets must be accurately known to laterally position and simultaneously pick up the pair of glass sheets from the conveyor if the conveyor operates continuously.